Here below, the drawbacks of the prior art are illustrated through the particular case of a narrow-band radio-communications system compliant with the GSM standard.
GSM radio access technology is a second-generation mobile telephony technology used for the transmission of voice as well as low-volume digital data, for example, SMS (Short Message Service) texting messages or MMS (Multimedia Message Service) messages.
The area served by a mobile telephony operator of the GSM network is divided into small zones, also called cells.
Each cell has a certain number of narrow-band frequency channels associated with it (for example the GSM-P 900 MHz band comprises 124 duplex channels, the width of each channel being 200 kHz). For each cell, from the set of frequency channels associated with it, we define a respective beacon channel that contains synchronization channels (especially the FCCH or Frequency Correction Channel, SCH or Synchronization Channel, BCCH or Broadcast Control Channel) enabling the radio-communications terminals to detect the presence of base stations.
To communicate, the radio-communications terminal sets up a radio link with one or more base stations. From the base station, the communication then passes to the radio-communications management centre through microwave links or lines.
The beacon channel (and therefore the synchronization channels) are carried by a beacon frequency on which a modulated signal of constant power is sent out constantly, enabling the radio-communications channels to carry out, for example, power measurements.
The beacon frequency has a twofold role: a role of an analog beacon frequency (for frequency and time adjustment and power measurement) and a role of a digital beacon frequency (providing “system” information to the corresponding cell).
The beacon frequency is sent out continuously (by the corresponding base station) and conveys a variety of data in TDMA (Time Division Multiple Access) mode on a particular time slot, especially synchronization data and Public Land Mobile Network (PLMN) subscription network identification data.
The beacon frequency can be identified by an ARFCN (Absolute Radio Frequency Channel Number).
A radio-communications terminal is capable of examining the beacon frequencies of different cells when it is powered on or when it is a standby state or when a communication link is set up.
Classically, when a GSM terminal wishes to communicate through the GSM network, it must search for a beacon frequency and decode it (the decoding of a beacon frequency enables it to confirm that it is situated in a cell of the GSM network) and thus get synchronized with the base station generating this beacon frequency.
In the context of this synchronization, in a first phase, the terminal carries out a measurement of the reception power of all the beacon frequencies that it receives by scanning the entire GSM frequency band with a 200 kHz step (for example the GSM-P 900 MHz band which extends from 890 MHz to 915 MHz includes 124 possible beacon frequencies associated with 124 duplex channels). At the end of this scanning operation, the terminal obtains a table of the frequencies belonging to the GSM band and their respective associated power values. Thus, the terminal has a power spectrum 10 (as illustrated in FIG. 1) associating, with each frequency of the frequency table, a power value representing the power with which the frequency is received by the terminal. In FIG. 1, the ARFCN numbers of the beacon frequencies 11 received by the terminal are represented on the x-axis and the reception power levels 12 (expressed in decibels) are represented on the y-axis.
In a second phase, the terminal sorts out the beacon frequencies measured and arranges them in descending order of reception power level. Thus, the terminal obtains a classification of beacon frequencies sorted out by descending order of reception power.
Then, in a third phase, the terminal tries out, one by one, the beacon frequencies sorted out in descending order of reception power until it is able to decode a beacon frequency (i.e. obtain the synchronization and identification data conveyed by this beacon frequency) and thus get synchronized with this beacon frequency.
There are situations where a narrow-band radio-communications system can be disturbed by one or more broadband radio-communications systems.
For example, in the United States, the GSM system is disturbed by broadband radio access systems, for example, of the CDMA type. Thus, certain frequency ranges of the GSM band are liable to be used by CDMA radio access systems.
In such situations, the major drawback of the classic synchronization technique (described here above) lies in the fact that the GSM terminal will attempt to get synchronized on frequencies liable to be used by broadband radio access systems whereas these frequencies are not “true” beacon frequencies of the GSM network, i.e. they convey no synchronization information by which the terminal can get synchronized with the GSM network. This therefore leads to a loss of time for the effective synchronization of the GSM terminal on a beacon frequency of the GSM network.
For example, taking the width of a GSM communications channel to be 200 kHz, if a broadband radio access technology is used on 8 MHz of the GSM band, then this broadband radio access technology uses 40 frequencies (i.e. 8 MHz/0.2 MHz) of the GSM band. Assuming that the GSM terminal takes about 1.5 seconds per frequency decoding attempt, the attempt to decode these 40 frequencies may lead to a loss of time of about 60 seconds (i.e. 40×1.5 seconds) in the process of synchronization of the terminal, and may even make synchronization impossible.
However, in mobile communications, it is important that the synchronizing of a radio-communications terminal with a cell should be as speedy as possible in order to provide the user with a communications link as soon as possible.
Furthermore, another drawback of these decoding attempts made on non-decodable beacon frequencies is that they give rise to unnecessary power consumption by the radio-communications terminal, which needs to conserve the power that it has available to it.